I wouldn't be surprised if one reason we are talking to Barcelona is because Thiago wants to try and leave on as good terms as possible. Yes, so he doesn't burn his bridges in case one day he wants a return, but also because he does feel an attachment to the club, and perhaps as if he still has something to prove.
That might sound like him trying to have his cake and eating it but this is a big decision for him. One of the the catalan TV commentators asked if he wanted "games or glory" and made it sound like he was chickening out of the challenge. Presumably the club are saying the same.
Unfortunately the Barca board can't be seen to give in too easily. Us enforcing the release clause is OK. If they let him go willingly, they need something, preferably a buyback clause. Something to say they tried to keep him, but shortterm goals and money won out.
I can see why that might give United's negotiators a problem. I do think we're still in the driving seat, but as I've always said, I don't think we're the only club that wants him.